1. Technical Field
The present invention relates in general to artificial lift for oil and gas wells and, in particular, to an improved system method and apparatus for artificial lift deployed on concentric tubing that allows gas to be vented from below packers in an oil and gas well.
2. Description of the Related Art
In oil and gas wells, artificial lift systems (e.g., pumps) are used to extract the fluid from the wells. In some wells, the pumps are set in the casing below the packers where it is typical to only draw down the wells to no more than the “bubble point.” The bubble point is the pressure at which gas vapor first separates from a fluid. When the pressure remains above the bubble point, gas does not form below the packer and therefore eliminates the need to vent gas from below the packer. Another conventional design provides a vented packer that allows the gas to escape to the annulus between the casing and the production string that supports the pump so that the gas may be vented to the wellhead at the surface. Venting gas in the annulus, however, results in an inventory of gas build up within the annulus that is typically not considered desirable or acceptable.
FIG. 1 depicts another conventional option where a dual production string extends from the wellhead to the packer 115. One conduit 133 carries the produced fluids from the artificial lift (e.g., motor 121, seal 123, gas separator 127 and pump 125) and another conduit 135 vents the gas from below the packer 115 to the surface. A packer penetrator 116 provides a path for an electrical cable 118 to the motor 121. Both production strings 133, 135 can be fitted with sub-surface safety valves 136 if required.
In well applications where dual strings, vented packers or some alternative solution cannot be used, the artificial lift system is exposed to potential gas lock when “free gas” levels at the pump intake are sufficiently high. This problem forces the well operator to limit the well drawdown and, consequently, the production rate so the bottom hole pressure at the pump remains above the fluid bubble point, thereby preventing a build up of gas below the packer. Moreover, this limitation prevents the use of more efficient rotary or static gas separation techniques, thus limiting the maximum fluid production from the well.